The manufacture of electronic power modules requires materials to be used that have good electrical insulation properties together with good thermal conduction properties. In addition, the materials used must have a coefficient of expansion which is close to that of the semiconductor materials used, such as silicon. Further, it is naturally desirable for the material used in making the above-mentioned modules to be as cheap as possible.
Various different processes have already been proposed for bonding a sheet of metal, in particular copper, to an electrically insulating ceramic substrate, and in particular aluminum nitride.
For example, Document EP-A-0 123 212 describes a method of bonding a sheet of copper on an aluminum nitride substrate, which method consists in using heat treatment to cause a layer of alumina to grow on the aluminum nitride substrate, and then in placing a sheet of copper on the layer of alumina and bonding the sheet of copper to the layer of alumina by a method which is known per se.
More precisely, above-mentioned Document EP-A-0 123 212 teaches, in particular, bonding a sheet of copper with an AlN substrate by generating a layer of alumina on the substrate by exposing it to air at 1200.degree. C. for a period of two to five hours, then applying the sheet of copper and heating to 1078.degree. C. for 15 minutes (min) under a nitrogen atmosphere to obtain good connection by virtue of the resulting layer of Al.sub.2 O.sub.3 -Cu.sub.2 O.
Although this technique is currently widely used in industry, it does not give full satisfaction.
The copper-alumina bond is often unsatisfactory. The copper sheet can frequently be torn off with ease.
An examination of products obtained in this way reveals the presence of bubbles beneath the copper sheet, thereby spoiling the mechanical strength of the bond.
For example, the following documents describe this bubble formation: 1) J. Am. Ceram. Soc. 72 [8]1322-27 (1989), pp. 1322-1327 Role of oxygen in bonding copper to alumina--Yuichi Yoshino; 2) Materials Research Society, Symp. Proc. Vol. 40, 1985, Eutectic bonding of copper to ceramics--Marc Wittner; 3) Mat. Res. Soc. Symp. Proc. Vol. 40 1985 Material Research Society, Directly bonded copper metallization of AlN substrates for power hybrids, Petra Kluge Weiss et al.; 4) IEEE 0569, 5503, 89, 0029, Behavior of aluminum nitride ceramic surfaces under hydrothermal oxidation treatments, D. Suryanarayana et al.
In addition, examining products obtained by performing the method taught in Document EP-A-0 123 212 generally reveals the presence of cracking in the alumina layer. Such cracking is mentioned in particular in the document IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. 12, No. 3, Sep. 1989, The influence of moisture on surface properties and insulation characteristics of AlN substrates, Yasutoshi Kurihara et al.
A great deal of research has already been performed in an attempt to improve the bonding between a metal sheet and aluminum nitride.
In particular, attempts have been made to observe the influence of alumina layer thickness on bubble formation by performing numerous tests with alumina layers of different thicknesses. In general, these tests have shown that the number and volume of the bubbles increases with increasing thickness of the alumina layer. Persons skilled in the art have thus attempted to make devices having a thin alumina layer. However, these tests have not given satisfaction. They give rise to an unsatisfactory bond.
Attempts have also been made to observe the influence of heat treatment temperature on bubble formation, in particular the temperature during the oxidizing stage. These tests have not given decisive results.
Finally, other tests have attempted to show the influence of water vapor on the overall bonding process. For example, such tests are described in the documents Ann. Chim. Fr. 1985, 10, pp. 79-83, entitled (in translation) Behavior of sintered aluminum nitride in water vapor at high temperature by Y. Yefsah et al.; and 7th European Hybrid Microelectronics Conference, Hamburg, May 1989, Surface treatment of AlN substrate, Yoshirou Kuromitsu et al. In outline, these tests have thus far merely indicated that the presence of water vapor makes it possible to increase the speed of oxidation. That is why the person skilled in the art has been inclined up till now to use a somewhat damp atmosphere during oxidation heat treatment in order to shorten the overall process.
In order to eliminate bubbles, the person skilled in the art is thus generally constrained to make grooves in the surface of the metal sheet to serve as drainage channels, as taught in Document DE-A-3 324 661, for example. It would appear that such grooves do indeed limit air bubble formation at the metal/substrate interface. However, the above-mentioned grooves facilitate unwanted diffusion of etching agents between the metal sheet and the substrate during any subsequent subtractive method of obtaining a specific pattern of metal areas.
In conclusion, the techniques proposed so far for bonding a sheet of copper to a substrate of electrically insulating material have not given full satisfaction.
An object of the present invention is to provide a novel method of bonding a sheet of metal, preferably copper, onto an aluminum nitride substrate.